The long-term goal of the project is to understand the mechanisms underlying the regulation of uterine blood flow during pregnancy, which is important for both fetal development and maternal cardiovascular well-being. The proposed studies will focus on the adaptation of contractile mechanisms of the uterine artery to pregnancy. Pregnancy decreases uterine artery smooth muscle tone, but increases its acute contraction to adrenergic stimulation. We demonstrated that pregnancy enhanced the role of extracellular signal-regulated kinase (ERK) in alpha1-adrenoceptor-mediated contractions and in suppressing protein kinase C (PKC)-mediated contraction by the uterine artery. PKC plays an important role in regulating sustained contraction of smooth muscle, and hence, vascular tone. In addition, PKC produces a negative feedback regulation on alpha1-adrenoceptor-mediated contractions in vascular smooth muscle. These findings lead to the proposed studies of mechanisms, testing the main hypothesis that pregnancy up-regulates the ERK pathway and down-regulates the PKC pathway in uterine artery smooth muscle resulting in decreased basal vascular tone with an increase in contractile capability. Three of its main corollaries will be addressed by 3 Specific Aims which will test whether 1) pregnancy up-regulates the ERK pathway resulting in down-regulation of the PKC pathway in the uterine artery, 2) the attenuated PKC pathway plays a key role in pregnancy-increased uterine artery contractile capability by enhancing pharmacomechanical coupling of alpha-adrenoceptors, 3) the attenuated PKC pathway plays a key role in pregnancy-decreased uterine artery basal vascular tone by suppressing Ca2+ sensitivity of contractile myofilaments. To achieve these aims, we propose a series of experiments in the uterine arteries from nonpregnant and near-term (about 140 days) pregnant sheep. We will measure expression and activities of ERK and PKC isoenzymes, a1-adrenoceptors, inositol 1,4,5-trisphosphate (IP3) synthesis, IP3 receptor affinity and density, intracellular free Ca2+ concentration, and Ca2+ sensitivity of contractile myofilaments. The results will provide a comprehensive and novel assessment of the unique effect of pregnancy on the ERKIPKC pathway, and a quantitative assessment of the dual role of PKC in the regulation of alpha1-adrenoceptor-mediated pharmaco-mechanical coupling and myofilament Ca2+ sensitivity as determinants of vasoreactivity of the uterine artery. Such an understanding has obvious clinical implications because the maladaptation of uterine circulation to pregnancy is associated with fetal developmental abnormalities and maternal cardiovascular disorders.